Re the WheelTug item:
This was first done by Consolidated Diesel Electric of Canada in the mid-1950s. As its company name implies, this was a diesel-powered unit that attached to and drove the main wheels of the aeroplane and thus did not depend on any aircraft systems. Presumably, it did not catch on because of the then low price of jet fuel.
abtn replies:
Nothing is new, the fax is over 100 years old. Clearly it did not catch on for the reasons you have given.
My understanding is that WheelTug is a 21st century version. Thank you for your interest.